1. Field of the Invention
The present invention relates generally to a miniature wideband surface mount bias tee network for use with radio frequency (RF) and microwave frequency signals, and more particularly to a bias tee network where miniaturization has been achieved through techniques including vertical stacking of components while still permitting use with automated pick and place assembly equipment.
2. Description of the Prior Art
A bias tee circuit is used with radio frequency and microwave frequency signals to couple a direct current (DC) voltage onto a line used for alternating current (AC or RF) signals. A bias tee can also separate a combined RF and DC signal into separate RF and DC signals. Additionally, a bias tee can remove the DC component of a composite signal in order to isolate an RF component.
A bias tee generally has three ports, DC, RF and RF+DC. A low frequency or DC signal is applied to the DC port. A high frequency or RF signal is applied to the RF port. A combined signal results at the RF+DC port. The standard bias tee has an inductor functioning as an RF choke and a capacitor used for DC blocking. Generally, the inductor is wire wound on a ferrite core. The inductor has one end connected to the DC port and another end connected to an internal node. The internal node joins the DC blocking capacitor with the RF+DC port. The other end of the capacitor is connected to the RF port.
U.S. Pat. No. 7,012,486 entitled MINIATURE WIDEBAND BIAS TEE teaches a miniature wideband bias tee having a small package size. The bias tee has a low temperature co-fired ceramic (LTCC) substrate composed of multiple layers, each with circuit features. A first inductor is located within the substrate. A second inductor is attached to the top surface and has a core with a wire wound on the core. The second inductor serves to increase the usable frequency bandwidth of the bias tee. Terminals are located on the top and bottom surfaces of the LTCC substrate. Ends of the second inductor core wire are connected to terminals on the top surface. A capacitor is mounted to terminals on the top surface. Several vias extend through the substrate and electrically connect the terminals to the inductor and capacitor. This bias tee provides a small package, but the frequency response is limited to a range of 50 MHz to 6 GHz. Further, manufacturing this bias tee requires multiple layers of LTCC to be custom made, assembled and then fused, which requires very specialized manufacturing equipment. Additionally, a second inductor is required, further increasing part and assembly cost. A major challenge is to vertically stack components without incurring excessive cost.
Another method of producing an inductor which is vertically stacked to avoid requiring additional horizontal space is taught by U.S. Pat. No. 7,868,431 entitled COMPACT POWER SEMICONDUCTOR PACKAGE AND METHOD WITH STACKED INDUCTOR AND INTEGRATED CIRCUIT DIE. Herein, a miniature inductor is made with an inductor core disposed on or in a substrate, and a combination of several half-coil forming conductive elements disposed beneath the inductor core and several half-coil forming conductive elements typically made of bond wires disposed atop the inductor core. The upper half-coil forming elements are conductively connected to respective lower half-coil fanning elements to jointly form an inductive coil enclosing the inductive core. While this method of forming a stacked inductor does save on horizontal real estate, there are numerous specialized manufacturing steps needed to form elements within the substrate, and numerous wirebonding operations needed as well. Additionally, the circuit must be fully encapsulated to protect it from the environment and to permit handling.
U.S. Pat. No. 5,936,840 entitled STACKED PASSIVE COMPONENTS teaches an edge-mounting method of using vertical space to save on horizontal space. Rectangular components such as surface mount resistors and capacitors are stacked on end with insulating spacers therebetween. The existing end terminals of each surface mount component provide many of the electrical contacts needed as well as solderable mounting points for mechanically joining the resulting module to a printed circuit board, and conductive pathways are formed in secondary operations along the resulting module's other faces as necessary for electrical circuit interconnections. However, this stacking methodology lends itself best to uniformly sized rectangular components, but poorly to the sort of wound coil inductors best suited for high frequency signals. Further, specialized tooling and machinery is needed to assemble the multiple layer stack and to form the conductive pathways along the module's faces.
A more cost-effective miniaturized bias tee providing wide-band high frequency performance would be fabricated on an inexpensive and readily available substrate such as pc board material, have circuit elements that are readily available or simply manufactured, require only simple and cost-effective assembly operations, and be surface mountable with standard automated pick-and-place surface mount assembly equipment.